Scanning probe lithography techniques can be employed to pattern self-assembled monolayers (SAMs) at the sub-micron length scale (Ross et al. (1993) Langmuir 9: 632; Schoer et al., (1996) J. Phys. Chem. 100: 11086; Zamborini & Crooks, (1998) J. Am. Chem. Soc. 120:9700; Xu & Liu, (1997) Langmuir 13: 127; Xu et al., (1998) J. Am. Chem. Soc. 120: 9356; Piner et al., (1999) Science 283: 661; Horib et al., (1999) Science 286: 523; Gorman et al., (2000) Langmuir 16: 6312; Xu et al., (1999) Langmuir 15: 7244; Maoz et al., (1999) Adv. Mater. 11: 55; Maoz et al., (2000) Adv. Mater. 12: 424). For example, Crooks and co-workers produced patterns in SAMs on gold by selectively removing the alkanethiolates with a scanning tunneling microscope (STM) (Ross et al, (1993) Langmuir 9:632; Schoer et al., (1996) J. Phys. Chem. 100:11086; Zamborini & Crooks, (1998) J. Am. Chem. Soc. 120: 9700). Recently, other SAM patterning techniques such as nanografting (Xu & Liu, (1997) Langmuir 13: 127; Xu et al., (1998) J. Am. Chem. Soc. 120: 9356), and dip pen nanolithography (Piner et al., (1999) Science 283: 661; Hong et al., (1999) Science 286:523) have been described. More recently, an STM based replacement lithography technique was disclosed, in which SAM thiolates are selectively desorbed from the gold substrate and replaced with a second alkanethiol in solution (Gorman et al., (2000) Langmuir 16: 6312).
Chemical gradients transport materials in a directional manner, and are responsible for driving many important biological and physical processes. For example, the growth of axons from ganglions to target tissues and the directed movement of certain bacteria toward nutrients occur in response to concentration gradients of molecules emanating from axon target or food source (chemotaxis) (Ruardy et al., (1997) Surf Sci. Rep. 29: 1). Concentration gradients of molecules in fluids or on surfaces also affect a variety of phenomena including osmotic swelling, surface pressure, and surface wettability. Efforts to establish and manipulate these parameters are ongoing in the art, as are efforts to develop new methods of transporting fluids in microchannels and new transport paradigms for the fabrication of chip-based chemical devices (Gallardo et al., (1999) Science 283: 57).
Surface-bound chemical gradients have previously been produced on ;millimeter to micron length scales (Ruardy et al., (1997) Surf. Sci. Rep. 29: 1; Gallardo et al., (1999) Science 283: 57; Chaudhury and Whitesides, (1992) Science 256: 1539; Daniel et al., (2001) Science 291: 633; Liedbera and Tengvall, (1995) Langmuir 11: 3821; Liedberg et al., (1997) Langmuir 13: 5329; Lestelius et al., (1999) Colloid Surface B 15: 57; Terrill et al., (2000) J. Am. Chem. Soc. 122: 988). In some cases, these gradients have been employed in directional transport. For example, Whitesides et al. fabricated SAM gradients consisting of decyltrichlorosilane on silicon substrates using a diffusion controlled vapor deposition technique. Water droplets were observed to travel uphill under the influence of the resulting spatial gradient in the surface free energy (Chaudhury and Whitesides, (1992) Science 256:1539). Liedberg et al. prepared millimeter scale SAM gradients on gold surfaces by cross-diffusing two different alkanethiols from opposite ends of a polysaccharide matrix (Liedberg and Tengvall, (1995) Langmuir 11: 3821; Liedberg et al., (1997) Langmuir 13: 5329; Lestelius et al., (1999) Colloid Surface B 15: 57).
None of the described studies, however, have employed STM-based replacement lithography to fabricate a surface-bound static or dynamic gradient. Moreover, prior to the present disclosure, the concept of static and dynamic gradients has not found application in the field of molecular machines. The ability to fabricate a surface-bound chemical gradient is desirable and can be employed in a range of applications. For example, a gradient so formed can be employed to construct a molecular machine, such as a particle synthesizer, as well as another machines and devices. STM-based gradient fabrication methods are also desirable, due to the relative availability of STM apparatuses that can be employed in the fabrication of gradients, as well as in the fabrication of other components of a molecular machine. To date, neither a static nor a dynamic STM-generated gradient on a nano- or micro-scale has been disclosed, nor has a gradient prepared in this fashion been disclosed as a component of a molecular machine. The present invention solves these and other problems.